According to ASTM D883, rigid resinous compositions are defined as plastic materials which have a stiffness or apparent modulus of elasticity greater than 100,000 pounds per square inch at 23.degree. C., when determined in accordance with The Method of Test for Stiffness and Flexure of Plastics (ASTM D747). In accordance with the same test, semi-rigid plastic materials are defined as having a modulus of elasticity greater than 10,000 pounds per square inch at 23.degree. C. and flexible plastic materials are defined as having a modulus of elasticity less than 10,000 pounds per per square inch at 23.degree. C.
Flexible or semi-rigid polyvinyl halide resin compositions are useful in a wide variety of applications due to the durability and resistance to solvents of the compositions. These applications include manufacture of articles derived from these compositions which are useful in the automotive, construction and appliance industries. Articles derived from rigid polyvinyl halide compositions are readily coated, printed on or painted and exhibit good adhesion of the coating and good permanence of appearance.
Conventional melt-processed flexible or semi-rigid polyvinyl halide resin compositions contain a plasticizer which is susceptible to migration. Internally plasticized PVC copolymer latexes are also well established. These aqueous dispersions are cast into films where the polymer coalesces from water to form a film during drying. Solution PVC and organosols of PVC are known, these are used in slush molding or spread coating. The PVC fuses on application of heat. Plastisols are known, and fusion takes place after casting, dipping or blow molding by subsequent heating. The present invention is directed not to these forms of PVC but to melt-processed PVC. That is, PVC compositions which are compounded in the melt state, and formed or applied to articles in the melt state by the use of melt processing techniques, further elaborated hereinbelow.
Various approaches to improve migration properties of flexible or semi rigid polyvinyl halide compounds are known. U.S. Pat. No. 3,892,692 to Heiberger discloses flexible vinyl chloride-ethylene copolymer compositions blended with a plasticizer and having a low permanent modulus index, thus, more expensive polymeric plasticizers are avoided. The vinyl chloride-ethylene copolymers are internally plasticized and lower amounts of inexpensive external plasticizer are required to obtain a flexible product. By the use of low cost plasticizers containg 20 to 42 major atoms, the resulting vinyl chloride-ethylene copolymer compositions exhibit a high degree of plasticizer permanence. The compositions of Heinerger being blends of plasticizer and (polyvinyl chloride-ethylene) with ethylene being, a non-polar comonomer, however, contain external plasticizer and were observed to exhibit migration although to a lesser extent.
PVC-acrylate copolymers are disclosed by Haller in U.S. Pat. No. 4,752,639 pertaining to grafted embodiments. In particular, Haller teaches flexible to soft shaped articles containing a vinyl chloride-acrylic acid ester graft copolymer containing 20 to 60% by weight of acrylic acid ester polymer and 40 to 80% by weight of vinyl chloride and 0 to 30% by weight of other ethylenically unsaturated comonomers copolymerizable with vinyl chloride grafted thereon. Compounds utilizing conventional additives are demonstrated. Haller teaches the graft copolymer of a preferred embodiment together with stabilizers conventionally used in PVC technology exhibits an ideal pattern of properties for many applications. Thus, the products are distinguished by good flexibility without plasticizer migration being able to occur; they possess excellent low temperature properties; are stable to aging and weathering and have good oil resistance; and the surfaces are smooth and homogeneous. Furthermore, the graft copolymer can without any problems be converted into shaped articles, especially sheet-like structures, which can readily be glued and printed, can be high frequency welded, are readily thermoformable and are distinguished by tensile strength, good tear strength, good elongation at break and good tear propagation resistance. Haller demonstrates blends of these graft copolymers and various other thermoplastic polymers such as VAE, PMMA, PV, polyether copolymers, PVDF and SMA among others.
The improvement of Heiberger is noteworthy within the scope of a free standing, unitary molded article such as a sheet, film, or mechanical part made from the copolymer. However, when a copolymer having migratable plasticizer is painted or the copolymer is incorporated on or about a dissimilar substrate, interfacial adhesion is critical. Permanence of adhesion is compromised by the migrating component. Such exemplary multi-component articles are PVC and a substrate, such as vinyl coated metal, vinyl cladding on wood, and co-extrusion or co-injection molding with another plastic. Any migration over long periods will affect adhesion in addition to effects on paint compositions incorporated thereon.
The use of plasticized PVC compositions present limitations apart from adhesion parameters and plasticizer migration. Over the years PVC compounds have predominated in wire and cable insulation. These compounds contain typically polyvinyl chloride resin, plasticizer, filler typically clay), and heat stabilizer, with lubricants, pigments and other optional modifiers. Electrical grade polyvinyl chloride resin must have a high electrical resistivity, which can be expressed in terms of a high volume resistivity, in excess of 10.sup.11 ohm-cm and preferably in excess of 10.sup.12 ohm-cm, measured in water at 60.degree. C. This property applies to all classes of wire.
Many commonly used stabilizers and plasticizers cannot be used in melt-processed electrical grade polyvinyl chloride resin compositions because they reduce the electrical resistance. For example, plasticizers such as tricresyl phosphate and dioctyl phthalate reduce volume resistivity in inverse proportion to the amount of such plasticizer present. A semirigid or flexible melt-processible PVC, absent plasticizer and exhibiting volume resistivity in excess of 10.sup.11 Ohm-cm, with improved permanent adhesion to metal conductors and having inherent non-migrating characteristics would thus be commercially attractive.
Plasticizers present problems in coating of plasticized melt-processed PVC with over-varnishes, lacquers or paint. Under normal use conditions this plasticizer can migrate out of the polyvinyl halide resin composition and attack the coating. If a paint composition not derived from plasticized PVC itself were applied to the composition, this plasticizer migration into the paint can attack the composition, and marring of the weakened coating as well as loss of its adhesion to the surface of the composition occurs. This is unacceptable for durable, appearance articles. Consequently, the use of flexible or semi-rigid melt-processed polyvinyl halide compositions for applications where the ability to be painted is desired are discouraged despite the benegits that the polyvinyl halide compositions would otherwise provide to the application.
In view of these disadvantages, it would be desirable to have melt-processible flexible or semi-rigid polyvinyl halide compounds absent external plasticizer which migrates or interferes with adhesion to dissimilar substrates or paint coatings, thereby providing compositions which permanently adhere to substrates, retaining high adhesive strength and on which the surface can be painted and will retain paint adhesion, strength and appearance of the paint over long periods of use. Flexible or semi-rigid melt-processed polyvinyl halide compositions exhibiting high specific adhesion to polar metal, polymeric or wood substrates would also be commercially desirable as clear or pigmented protective coatings, including extruded or molded claddings over these substrates.